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<ul><li><p>www.elsevier.com/locate/wasman</p><p>Waste Management 27 (2007) 3043</p><p>Operating problems in anaerobic digestion plants resultingfrom nitrogen in MSW</p><p>Klaus Fricke a, Heike Santen a,*, Rainer Wallmann b, Axel Huttner b, Norbert Dichtl c</p><p>a Technical University of Braunschweig, Leichtwei-Institute, Department of Waste Management, Beethovenstr. 51a, 38106 Braunschweig, Germanyb IGW Ingenieurgemeinschaft Witzenhausen Fricke and Turk GmbH, Bischhauser Aue 12, 37213 Witzenhausen, Germany</p><p>c Technical University of Braunschweig, Institute for Sanitary Engineering, Pockelsstr. 2a, 38106 Braunschweig, Germany</p><p>Accepted 7 March 2006Available online 24 July 2006</p><p>Abstract</p><p>Organic waste and municipal solid waste usually contain considerable amounts of different nitrogen compounds, which may inhibitanaerobic degradation processes and cause problems in the downstream and peripheral devices. This refers particularly to the differentprocess stages of anaerobic digestion, to wastewater treatment, and to exhaust air treatment.</p><p>Neither the knowledge about nitrogen problems nor the technologies for elimination of nitrogen compounds from the wastewater orthe exhaust air of anaerobic digestion can be regarded as state-of-the-art. Most of the technologies in question have already been appliedin other areas, but are barely tested for application in anaerobic digestion plants. The few performance data and experiences at hand weremainly derived from pilot and demonstration facilities.</p><p>In this paper, the problem of nitrogen will be discussed in detail according to the separate problem fields based on the authors expe-rience, as well as on the basis of a review of the relevant literature. Furthermore, possible solutions will be proposed and the need forfurther research and development will be formulated. 2006 Elsevier Ltd. All rights reserved.</p><p>1. Introduction</p><p>Due to the relatively low costs, the high flexibility of theprocess and the possibility of centralized and decentralizedapplication, mechanical-biological waste treatment (MBT)processes are gaining importance, not only in Germany. Inthis context, anaerobic digestion for municipal solid wastetreatment is becoming increasingly interesting due to itsadvantages in terms of energy production and exhaustemissions compared to aerobic procedures. Nevertheless,anaerobic digestion has not yet been able to establish itselfon the market to the same extent as aerobic technologies.Apart from the higher investment costs for anaerobicdigestion plants in comparison to aerobic treatment plants,this is also due to the fact that anaerobic digestion is stillconsidered to be less stable in operation. Moreover, opera-</p><p>0956-053X/$ - see front matter 2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.wasman.2006.03.003</p><p>* Corresponding author. Tel.: +49 531 391 3958; fax: +49 531 391 4584.E-mail address: heike.santen@tu-bs.de (H. Santen).</p><p>tional problems are more difficult to remedy, once theyhave occurred.</p><p>One important source of operational problems is thenitrogen compounds, which enter the process with the feedmaterial. Table 1 shows the nitrogen content of differentorganic waste, sewage sludge and municipal solid waste.In the untreated raw waste, the nitrogen is predominantlyorganically bound.</p><p>Nitrogen may cause problems in anaerobic digestionbecause of its metabolic products:</p><p> Ammonia (NH3), Ammonium NH4 , Dinitrous oxide (N2O), Nitrite NO2 , Nitrate NO3 .</p><p>Fig. 1 shows the process areas and material stages inwhich nitrogen compounds may cause problems.</p><p>mailto:heike.santen@tu-bs.de</p></li><li><p>Table 1Physicalchemical parameters and nutrient contents of selected organic waste materials</p><p>H2O (% FM)b Organic dry</p><p>matter (% DM)cNtotal (% DM)</p><p>c P2O5 (% DM)c K2O (% DM)</p><p>c CaO (% DM)c MgO (% DM)c</p><p>Organic waste 5280 3481 0.62.1 0.31.5 0.62.1 2.26.8 0.21.7Green waste (soft organic) 4880 3270 0.31.9 0.41.4 0.41.6 0.77.4 0.31.2Green waste (tree cuttings) 2552 6585 0.10.4 0.1 0.30.5 0.51 0.10.15Sewage sludge (digested) 6585 1540 4.05.3 4.75.2 0.30.5 5.78.2 0.81.2Bark 4575 6085 0.20.6 0.10.2 0.31.5 0.41.3 0.10.2Kitchen waste 7595 1.01.5Grape pomace 7575 1.52.5 0.81.2 3.45.3 1.42.4 0.21Fruit pomace 7080 9095 1.1 0.2 0.6 1.57 1.1 0.2Rumen contentsa 1020 8090 1.31.2 1.11.6 0.50.6 2.0 0.6Paper 2530 6279 0.20.8 0.150.6 0.020.1 0.51.5 0.10.4Draff/masha 9095 9095 0.81.8Yeast residues 4060 9095 1.42.0Residual household waste</p><p>(after separate collection)3545 5070 0.71.3 0.81.4</p><p>a Kuhn (1995) and Weiland (1999).b FM, fresh matter.c DM, dry matter.</p><p>Anaerobic Digestion </p><p>DewateringAerobic </p><p>post-treatment </p><p>Pre- Treatment Conditioning Conditioning Landfilling </p><p>Biogas</p><p>Incineration plant, RDF</p><p>Wastewater </p><p>Ammonianitrous oxide, </p><p>Ammonium/Ammonia</p><p>Ammonium/ Ammonia </p><p>Ammonium </p><p>Ammonium</p><p>Exhaust air </p><p>Fig. 1. Process areas and material stages of an anaerobic digestion plant possibly affected by nitrogen problems.</p><p>K. Fricke et al. / Waste Management 27 (2007) 3043 31</p><p>2. Biological process</p><p>The anaerobic digestion of organic matter is a complexprocess, which falls into four degradation steps. The spe-cific microorganisms that take part in the process have dif-ferent requirements on environmental conditions andmoreover coexist in synergetic interactions. Nitrogen playsan important role in anaerobic digestion: Nitrogen is neces-sary for the formation of new biomass. Furthermore, in theform of ammonium, nitrogen contributes to the stabilisa-tion of the pH value in the reactor. However, ammoniumin high concentrations may lead to the inhibition of thebiological process.</p><p>Microorganisms need nitrogen for the production ofnew cell mass, the absorption of nitrogen taking place inthe form of ammonium. The nutrient requirement is low,which is due to the low biomass formation. A nutrient ratioof the elements C:N:P:S at 600:15:5:3 is sufficient for meth-anisation. As the reduced nitrogen compounds are not</p><p>eliminated in the process, the C/N in the feed materialplays a crucial role. The C/N should range from 20 to 30in order to ensure sufficient nitrogen supply for cell produc-tion and the degradation of the carbon present in theprocess, and in order to avoid at the same time excess nitro-gen, which could lead to toxic ammonium concentrations(Weiland, 2001).</p><p>Ammonium is an important parameter for the buffercapacity in an anaerobic reactor. With concentrations ofup to 1000 mg/l, ammonium stabilises the pH value(ATV, 2002). Ammonium is released during the anaerobichydrolysis of organic nitrogen compounds, causing anincrease of the pH value. The ammonification thus coun-teracts the reduction of the pH value resulting from theacidification step of anaerobic digestion (ATV, 1993).</p><p>At a sufficiently high concentration, almost allsubstances inhibit anaerobic digestion (ATV, 1990). Itshould be noted that only the undissociated form of theintermediate catabolic product has an inhibiting effect on</p></li><li><p>Fig. 2. Dissociation balance between ammonia/ammonium depending on pH and on temperature (calculated according to Kollbach et al., 1996).</p><p>32 K. Fricke et al. / Waste Management 27 (2007) 3043</p><p>microorganisms. As the dissociation equilibrium dependson the pH and on temperatures in the reactor, which bothmay vary, it is difficult to provide detailed data on toxic orinhibiting threshold concentrations. The dissociation bal-ance of ammonia and ammonium, for instance, changesto ammonia with an increasing pH value and temperatureas shown in Fig. 2. From this follows that even smallchanges in the pH value are sufficient to cause an inhibi-tion. Furthermore, the bacteria may adapt themselves tohigh concentrations of certain substances, as long as theconcentration of the respective substance increases slowly.Because of this situation, it is difficult to determine an exactthreshold concentration that inhibits the process; ratherbroad ranges of possibly inhibiting concentrations can begiven.</p><p>The ammonia-induced inhibition occurs primarily dur-ing the anaerobic digestion of organic waste materials,which are rich in proteins, as ammonia nitrogen is releasedthrough the mineralisation of organic nitrogen compounds.The range of inhibiting concentrations of ammonia isbetween 30 and 100 mg/l (at pH value 6 7 and tempera-ture 6 30 C), whereas the respective concentrations ofammonium are between 4000 and 6000 mg/l (ATV, 1990).</p><p>The inhibition effects by different intermediate catabolicproducts can counteract each other. With an increasing pHvalue, for instance, the inhibition by hydrosulphide and byvolatile fatty acids declines, whereas the inhibition byammonium nitrogen increases. With the presence of certainsubstances, the inhibition impact may even be reversed. Inthe presence of hydrosulphide and carbon dioxide, forinstance, the dissociation balance of ammonia/ammoniumis displaced in the direction of ammonium and the inhibi-tion by ammonia is reversed (Knoche et al., 1996).</p><p>As mentioned above, microorganisms have the ability toadapt themselves to varying environmental conditions dur-ing a slow increase of, say, the ammonium or ammoniaconcentration in the reactor. Nevertheless, a suddenincrease in the ammonia concentration leads to an inhibi-</p><p>tion of the biological process. There are different emer-gency measures for the rapid recovery of the process, asfor example stopping the substrate supply, the additionof substrate with low nitrogen content, refeeding ofdigested material or lowering the pH value by addition ofacids. All of these measures can only eliminate low degreesof inhibition. In the case of a high degree of inhibition, theonly option is to empty the reactor and re-initiate the pro-cess. Therefore, close monitoring of the process is indis-pensable for early identification of inhibition effects.</p><p>In anaerobic digestion processes with intensive processwater recirculation, ammonium may accumulate in theprocess water and thus in the substrate for anaerobic diges-tion with the effects described above. In that case, furthermeasures for ammonium elimination may be necessary(see Section 4).</p><p>3. Exhaust air</p><p>The exhaust air emissions from waste treatment plantsplay a key role with regard to the acceptance by the popu-lation and the ecologic evaluation of the process. Nitrogencompounds that are relevant to the quality of the exhaustair are primarily ammonia and dinitrous oxide. In somecountries, exhaust air emission quality and particularlyemissions of odour and dinitrous oxide may be subject topermits.</p><p>In Germany, there are different legal licensing guidelinesfor the recovery of organic waste (biowaste), on the onehand, and for the treatment of municipal solid waste on theother:</p><p> For the construction and operation of plants for organicwaste treatment there are only essential requirements onexhaust air emission control, e.g., a minimum 300-m dis-tance of these plants from populated areas and defini-tion of maximum odour emissions of 500 odour units/m3 (German Technical Instruction on Air Quality</p></li><li><p>K. Fricke et al. / Waste Management 27 (2007) 3043 33</p><p>Control -TA-Luft; Anonymous, 2002). These require-ments can easily be fulfilled by structural measuresand the treatment of the exhaust air from aerated win-drows in a biofilter. In comparison to that, legal requirements on exhaust air</p><p>emission control from municipal solid waste treatmentare more detailed and stricter. The German 30th FederalEmissions Control Act (Anonymous, 2001) requires thecomplete encapsulation of the mechanical biologicalwaste treatment plants, including exhaust air collectionand treatment; it also defines limit values for some airpollutants, such as TOC, N2O and others (Table 2). Inorder to meet all of these requirements, a simple exhaustair purification in biofilters is not sufficient. As shown byresearch results and up-to-date operating experiences, itis necessary to treat the exhaust air in a thermal regen-erative oxidation plant (TRO) in combination with anacid scrubber (Wallman et al., 2001).</p><p>The relevant exhaust air emissions of nitrogen com-pounds are ammonia and dinitrous oxide emissions. Whileammonia contributes to the odour emissions and causesadverse effects on humans, the dinitrous oxide contributesto the anthropogenic greenhouse effect. Dinitrous oxideemissions can occur both during the intensive thermophilicphase of aerobic treatment/composting, and also duringthe aerobic post-treatment of solid digestion residues; thisis equally relevant for the treatment of organic and residualwaste.</p><p>Due to the mineralisation of organic nitrogen com-pounds under anaerobic conditions, the total nitrogen inthe solid digestion residue is mainly present as ammoniumand ammonia. In the first phase of aerobic post-treatment,most of it is stripped out in the form of highly volatileammonia. The reduction of the ammonia concentrations</p><p>Table 2Exhaust air emissions in comparison with the German limit values according2001)</p><p>German limit value(according to 30. BImSchV</p><p>Exhaust air (m3/ton waste input)(to treatment plant)</p><p>TOC (mg/m3)a 20/40b</p><p>TOC (g/Mg) 55Dinitrous oxide (g/ton waste input) 100Odour (odour units/m3)a 500</p><p>Polychlorinated dibenzo-p-dioxins(PCDD) and polychlorinateddibenzofurans (PCDF) (ng ToxicityEquivalents TE/m3)a</p><p>0.1</p><p>Dust (mg/m3) 30/10a</p><p>Ammonia (mg/m3)a</p><p>a Referring to standard cubic meters (0 C, 1013 bar).b Daily/half-daily mean.</p><p>in the waste by stripping with exhaust air leads to anincreased release of ammonia from the ammonium frac-tion, as the proportion of ammonia and ammonium is inbalance (see Fig. 2). In this way, up to 25% of the totalnitrogen in the waste may be stripped out into the exhaustair.</p><p>The main ammonia emissions take place during the firstweek of post-treatment and can amount to up to 1000 mg/scm (standard cubic meters), as shown by the investigationsof the aerobic post-treatment of digestion residues from aValorga plant (IGW, 2001). In that investigation, theammonia emissions declined in the course of further treat-ment and were about 100 mg/scm by the end of the thirdweek. Similar evolutions of the ammonia concentrationin the exhaust air were detected in other comparable inves-tigations (Fricke et al., 2001).</p><p>Very high peak emissions of ammonia may occur if thetemperatures in the windrows are high or if increasedammonium loads enter the aerobic post-treatment as aconsequence of high ammonium contents in the solid diges-tion residue itself or in the process water used for the irri-gation of the windrows. If the aeration is insufficient, highammonia concentrations in the atmosphere of the treat-ment hall may result that exceed the German limit valuefor exposure at working places of 50 ppm. According tocurrent operational experiences, only a strong, optimisedsuction aeration proved to be suitable in order to maintainthe contamination of the atmosphere in the treatment hallbelow critical levels.</p><p>The ammonia in the exhaust air may be effectivelyremoved through the use of an acid scrubber; the reductionrate can be close to 100%. Sulphur or nitric acids can beused as scrubbing acids, so that solutions of ammoniumsulphate or ammonium nitrate ar...</p></li></ul>

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